java - Question about Hibernate session.flush()

Question

I want to inquire about what actually the flush method does in the following case:

for (int i = 0; i < myList.size(); i++) {
    Car c = new Car( car.get(i).getId(),car.get(i).getName() );
    getCurrentSession().save(c);
    if (i % 20 == 0)
        getCurrentSession().flush();
}

Does this means that after the iteration 20, the cache is flushed, and then the 20 held memory objects are actually saved in the database ?

Can someone please explain to me what will happen when the condition is true.

Answer 1

From the javadoc of Session#flush:

Force this session to flush. Must be called at the end of a unit of work, before committing the transaction and closing the session (depending on flush-mode, Transaction.commit() calls this method).

Flushing is the process of synchronizing the underlying persistent store with persistable state held in memory.

In other words, flush tells Hibernate to execute the SQL statements needed to synchronize the JDBC connection's state with the state of objects held in the session-level cache. And the condition if (i % 20 == 0) will make it happen for every i multiple of 20.

But, still, the new Car instances will be held in the session-level cache and, for big myList.size(), you're going to eat all memory and ultimately get an OutOfMemoryException. To avoid this situation, the pattern described in the documentation is to flush AND clear the session at regular intervals (same size as the JDBC batch size) to persist the changes and then detach the instances so that they can be garbage collected:

13.1. Batch inserts

When making new objects persistent flush() and then clear() the session regularly in order to control the size of the first-level cache.

Session session = sessionFactory.openSession();
Transaction tx = session.beginTransaction();

for ( int i=0; i<100000; i++ ) {
    Customer customer = new Customer(.....);
    session.save(customer);
    if ( i % 20 == 0 ) { //20, same as the JDBC batch size
        //flush a batch of inserts and release memory:
        session.flush();
        session.clear();
    }
}

tx.commit();
session.close();

The documentation mentions in the same chapter how to set the JDBC batch size.

See also

• 10.10. Flushing the Session
• Chapter 13. Batch processing